(a) Fields of the Invention
The present invention relates to nitride semiconductor laser devices and fabrication methods of the same, and in particular to a nitride semiconductor laser device having a buried current blocking layer and a fabrication method of the same.
(b) Description of Related Art
Currently, a group III-V nitride-based compound semiconductor including group III elements of aluminum (Al), gallium (Ga) and indium (In) and a group V element of nitrogen (N), typified by gallium nitride (GaN) and represented by a general formula, InXGaYAl1-X-YN (wherein 0≦X≦1, 0≦Y≦1 and X+Y≦1), i.e., what is called a nitride semiconductor (hereinafter referred to as a GaN-based semiconductor), is regarded remarkable. With respect to, for example, an optical device, a light emitting diode (LED) using a nitride semiconductor is used in a large display device, a traffic light and the like. Also, a white LED obtained by combining an LED using a nitride semiconductor and a fluorescent material is partially commercialized and is expected to be substituted for currently used lighting equipment when the luminous efficiency is improved in the future.
Furthermore, a violet semiconductor laser device using a nitride semiconductor is now being earnestly studied and developed. As compared with a conventional semiconductor laser device emitting red or infrared light used for an optical disk such as a CD or a DVD, a spot diameter obtained on the optical disk can be reduced in using the violet laser device, and hence, the recording density of the optical disk can be improved.
The gallium nitride-based materials have excellent physical properties, such as a high breakdown electric field, a high electron saturation velocity in a high electric field, and a high two-dimensional electron gas density in a heterojunction, and therefore, are considered as highly potential materials for electronic devices.
In order to fabricate the above-mentioned device using nitride semiconductor, it is necessary to etch the nitride semiconductor into an arbitrary shape. In general, dry etching is employed for nitride semiconductor.
FIG. 7 is a sectional view showing a general structure of a blue-violet semiconductor laser device (referred hereinafter to as “a blue-violet LD”) using a GaN-based material (see Patent Document 1 (Japanese Unexamined Patent Publication No. 2003-142780)).
Referring to FIG. 7, the blue-violet LD includes a GaN substrate 101, an n-GaN layer 102, an n-AlGaN cladding layer 103, an n-GaN guiding layer 104, an MQW active layer 105 made of InGaN, a p-AlGaN overflow-suppressing layer 106, a p-GaN guiding layer 107, an n-AlGaN current blocking layer 109, a second p-GaN guiding layer 110, a p-AlGaN cladding layer 111, and a p-GaN contact layer 112.
The thus structured blue-violet LD is called a buried-type device. In this device, current does not flow through the n-AlGaN current blocking layer 109, but only flows through an opening from which the n-AlGaN current blocking layer 109 is removed. Light is confined by the refractive index difference between the n-AlGaN current blocking layer 109 and the second p-GaN guiding layer 110.
FIGS. 8A to 8C are sectional views showing process steps of a typical method for fabricating a buried blue-violet LD. Referring to FIG. 8A, the n-GaN layer 102, the n-AlGaN cladding layer 103, the n-GaN guiding layer 104, the MQW active layer 105 of InGaN, the p-AlGaN overflow-suppressing layer 106, the p-GaN guiding layer 107, and the n-AlGaN current blocking layer 109 are sequentially formed first on the GaN substrate 101 by a first growth by a MOCVD method.
Next, as shown in FIG. 8B, in order to form a current injecting area, part of the n-AlGaN current blocking layer 109 is etched to form an opening 120.
Thereafter, as shown in FIG. 8C, the second p-GaN guiding layer 110, the p-AlGaN cladding layer 111, and the p-GaN contact layer 112 are sequentially formed on the n-AlGaN current blocking layer 109 by a second growth by a MOCVD method to cover the opening 120, thereby fabricating a blue-violet LD.
In the above fabrication method, dry etching generally using a chlorine-based gas as an etching gas is employed for removing the part of the n-AlGaN current blocking layer 109. This dry etching, however, may damage the etched surface to cause degrade to the characteristics of the blue-violet LD. In particular, in the buried type structure, dry etching is performed on the portion that guides laser light, which disadvantageously leads to significant degradation of the threshold current for laser oscillation and the like.
One of etching methods causing less damage than the dry etching is a wet etching. However, nitride semiconductors are less etched with acids or alkalis in general.
Among them, a photoelectrochemical (PEC) etching is known as a method for wet etching the nitride semiconductor (see Patent Document 2 (Japanese Unexamined Patent Publication No. H10-93140) and Non-Patent Document 1 (Appl. Phys. Lett, vol. 72, No. 5, 2 Feb. 1998, p.p. 560-562)). FIG. 9 is a schematic view showing the PEC etching method. A nitride semiconductor layer 115 of GaN or the like connected to a platinum (Pt) cathode 114 is immersed in a potassium hydroxide (KOH) solution 113, and the layer 115 is irradiated with ultraviolet light from the outside to thus etch the nitride semiconductor layer 115.
The PEC etching etches n-type nitride semiconductor but does not etch p-type nitride semiconductor. Hereinafter, the mechanism of the PEC etching will be described with reference to FIGS. 10 and 11.
When an n-type nitride semiconductor is immersed in a KOH solution, it exhibits the band structure as shown in FIG. 10. In order to etch the nitride semiconductor by allowing GaN to react with holes and OH− as expressed by the following relation, the holes must be present at the interface between the nitride semiconductor and the KOH solution.GaN+3h++6OH−→2GaO33−+0.25N2+3H2O
As shown in FIG. 10, it is possible to accumulate holes at the interface with the band structure shown in this figure. However, there are fewer holes in the n-type nitride semiconductor. In view of this, the n-type nitride semiconductor is irradiated with light having a higher energy than the band gap of the n-type nitride semiconductor to generate holes at the interface, thereby promoting etching of the n-type nitride semiconductor.
When a p-type nitride semiconductor is immersed in the KOH solution, it exhibits the band structure as shown in FIG. 11. There are many holes in the p-type nitride semiconductor. However, since it has the band structure resistant to accumulating holes at the interface, no etching is observed even though it is irradiated with the light.